One field of this invention is the purification and recovery of diolefins, in particular C4 and C5 olefins such as butadiene and isoprene.
In the manufacture of ethylene and/or propylene by thermal cracking of naphtha, LPG, gas, oil, or fractions thereof, a hydrocarbon fraction containing conjugated diolefins may be obtained. From this fraction may be recovered a C4 fraction comprising 1,3 butadiene, n-butanes, n-butenes, isobutene, vinylacetylene, ethyl acetylene, 1,2-butadiene, and other C4's. A C5 fraction comprising isoprene, 1,3-pentadeine, cyclopentadeine, paraffinic compounds (e.g. isopentane and n-pentane) olefins (e.g. 2-methyl butene-1 and pentene-1), acetylenes (e.g. pentyne-1 and 1-pentene-4-yne), and other hydrocarbons (e.g. benzene, hexane, cyclopentane, cyclopentene and dicyclopentadiene) may also be recovered.
In diolefin purification, removal of acetylenic hydrocarbons is of particular importance. Diolefins such as 1,3 butadiene and isoprene are monomer raw materials for the production of synethetic rubber by catalytic solution polymerization. Acetylenes such as 2-methyl-1-butene-3-yne, inhibit the desired polymerization to reaction in concentrations of small as 300 ppm (parts per million). They react with polymerization catalysts, increasing catalyst consumption. For most solution polymerization work, acetylenes should be limited to 100 to 400 ppm maximum concentration, preferably less than 100 ppm.
Background information with respect to the subject process may be found in:
(1) "Techniques of Organic Chemistry" Volume 4, "Distillation", Second Edition, Interscience, particularly Chapter IV "Extractive and Azetotropic Distillation" by Carl S. Carlson and Joseph Stewart, Esso Research and Engineering Company, Linden, New Jersey and references noted thereafter.
(2) Chemical Engineering Progress, Volume 68, No. 10, October 1972 "Thermally Coupled Distillation - A Case History", W. J. Stupin, et al.
(3) Cahn, et al. U.S. Pat. No. 3,058,893 "Separation of Multicomponent Mixture In Single Tower", issued Oct. 16, 1962.
(4) August, 1969 Petro/Chem Engineer article by Dr. Thomas Reis "Compare Butadiene Recovery Methods (Processes, Solvents, Economics)."
(5) U.S. Pat. No. 4,134,795, issued Jan. 16, 1979 "Acetylenes Removable From Diolefin Streams By Extractive Distillation", inventor Colin S. Howat III. This patent contains an extensive listing of patents, as well as characterization thereof, related to extractive distillation.
While the subject improvements are particularly adaptable to the purification and recovery of diolefins, as previously mentioned, there are numerous other extractive distillation systems where the subject improvements and inventions may be applied. Without limitation, then, the following components to be separated and the solvent with which such would be extractively distilled are given: (1) Ethanol and water with solvent glycerol, (2) Acetone and methanol with water, (3) Propane and propylene with acrylonitrile, (4) Aromatic hydrocarbons and non-aromatic hydrocarbons with phenol, (5) Heptane and methylcyclohexane with aniline, (6) Benzene and cyclohexane with aniline, (7) Acetic acid and water with high boiling hydrocarbons.
It might be noted that the just listed systems have two primary components for separation and, thus, differ from the C4 and C5 hydrocarbon separation which would involve an overhead rafinate stream, a diolefin stream and an acetylene stream or the equivalent. The subject system, that is, is particularly adaptable to separation of two primary components or three. More cuts may be taken under special circumstances.
The subject of the present invention is a thermally coupled extractive distillation process which most effectively and efficiently carries out the conventional goals of the distillation systems above mentioned and those related thereto.